The present invention relates to a triple tube type projection television for producing a color image by converging blue, green and red images formed by respective projecting tubes using lenses and conforming these images onto a screen.
FIG. 1 shows an optical system of a conventional projection television of this kind. In this figure, a projecting tube 10G for projecting a green image (hereinafter "a green projection tube") is disposed in the center, and projecting tubes 10R and 10B for projecting red and blue images, respectively. (hereafter "red projecting tube" and "green projecting tube") are arranged in series on the left and right sides of green projecting tube 10G. In the construction of lenses group 20, lenses 20G, 20R and 20B are respectively arranged in front of green, red and blue projecting tubes 10G, 10R and 10B. Image light beams of green, red and blue projecting tubes 10G, 10R and 10B are converged by lenses 20G 20R and 20B and are focused onto screen 30. thereby producing a color image.
In the case above, central lens 20G is arranged such that optical axis 20Gs of lens 20G is perpendicular to screen 30 so that there is no problem with respect to this arrangement. However, left and right lenses 20R and 20B are arranged such that their optical axes 20Rs and 20Bs, respectively, are inclined with respect to screen 30. Therefore, as shown in FIG. 2, in accordance with the so-called Sheinpulf's law, tube faces (image faces) 10Ra, 10Ba of projecting tubes 10R, 10B, respectively, are inclined by angle .alpha. with respect to lens faces of lenses 20R, 20B, respectively, so as to focus the images on the entire screen 30 even when the above optical axes are inclined with respect to this screen. Inclination angle .alpha. between the lens face and the tube face is provided by the following formula when the magnification of lenses 20 is m and the angle formed between the lens face of lenses 20 and screen 30 is .theta.. EQU (1/m)tan.theta.=tan .alpha.
When the optical axes of lenses 20R, 20B on both sides of central lens 20G are set to be in conformity with the optical axis of central lens 20G on screen 30 as mentioned above, as shown in FIG. 1 left and right view angles .beta..sub.1, .beta..sub.2 of respective lenses 20R, 20B with respect to optical axes 20Rs, 20Bs thereof are different from each other so that the brightness of the red and blue colors are different from each other on screen 30 on the right and left sides thereof by the law of the fourth power of cosine of lens. Further, since the differences in brightness are reverse to each other on the left and right sides of the screen with respect to the red and blue colors, the color balance on screen 30 becomes so bad that an irregular color is caused.
Accordingly, another projection television as shown in FIG. 3 has been proposed internally by the assignee of the invention herein. In this projection television, the optical axes of the lenses on both sides of the central lens are respectively offset from the center of the screen so that left and right view angles .beta..sub.1, .beta..sub.2 with respect to respective optical axes 20Rs, 20Bs are approximately equal to each other.
In this projection television in FIG. 3, similar to the conventional projection television of this kind, optical axes 20Rs, 20Bs of lenses 20R, 20B are in conformity with the centers of tube faces 60R, 60B of projecting tubes. A so-called keystone correction for distorting images 70R, 70B produced on the projecting tubes in the shape of a trapezoid is performed to correct the difference in magnification between the left and right lenses on the basis of the above-mentioned Sheinpulf's law.
Image centers 80R, 80B are shifted from the centers of tube faces 60R, 60B in the respective projecting tubes such that centers (intersection points of diagonal lines of the trapezoids) 80R, 80B of picture images 70R, 70B are directed to the center of screen 30 in a state in which the optical axes of the lenses and the centers of the projecting tubes are in conformity with each other as mentioned above.
However, in such a projection television, the image centers are shifted from the centers of the tube faces by applying an external magnetic field applied to electrodes of the projecting tubes by centering magnets disposed in neck portions of the projecting tubes. This applies a biasing force to scanning electrons. When this biasing force is excessively applied to the scanning electrons, the image is disturbed in the moving direction thereof, thereby causing a halo phenomenon to occur.